Psoriasis is a lifelong skin disease. It can cause itchy or sore patches of thick, red skin with silvery scales. These patches are called plaques. Plaques can appear anywhere on your body, though are most commonly found on the elbows, knees, legs, scalp, lower back, face, palms of hands and soles of feet. People with psoriasis can develop pain, swelling, and stiffness in their joints. This condition is called psoriatic arthritis, or PsA.